The present invention relates to a read only memory (ROM), and more particularly to a circuit for repairing defects in a mask ROM. Mask ROMs have specific data stored therein during the manufacturing process. The mask ROMs are programmed during the manufacturing process to perform a specific operation and the predetermined data is stored by ion-implantation, contact switching, etc. A general mask ROM includes a redundancy memory cell array composed of redundant memory cells for replacing defective memory cells, repairing defective memory cells or programming data.
A conventional technique for repairing defects in a mask ROM has been disclosed in ISSCC, pp. 128-129, 1989 entitled "A 16Mb Mask ROM with Programmable Redundancy" by Yasuo Naruke, et al. The disclosed conventional defect repairing circuit uses as a redundancy memory cell a polysilicon fuse cell which can be electrically cut off. The defect is repaired by cutting off a selected fuse cell by allowing current flow therethrough. However, in this defect repairing technique, the size of the memory chip is increased since a polysilicon fuse cell is used as a redundancy cell. Accordingly, circuit integration is also decreased. Moreover, in the case where a normal memory cell array is not composed of fuse cells, an additional photoetching process is needed to form a redundancy memory cell array. Accordingly, the mask layers are essentially increased in number and the manufacturing cost becomes high.